The present invention relates to gratings made in optical fibres, and relates more specifically to gratings of the type called long-period gratings.
Long period optical fiber gratings may have a period of a few tens of microns (typically 30-40 microns), which show a low temperature dependence. Devices of this type are known in the art, as demonstrated, for example, by the paper by J. B. Judkins et al., xe2x80x9cTemperature-insensitive long-period fiber gratings,xe2x80x9d published on pages PD1-2 to PD1-5 of the Postdeadline Papers of the OFC Optical Fiber Communication ""96 conference held on Feb. 29, 1996, or the paper by V. Bhatia et al., xe2x80x9cTemperature-insensitive and strain-insensitive long-period grating sensors for smart structures,xe2x80x9d published in Opt. Eng. 36 (7), pp. 1872-1876 (July 1997).
These gratings, whose operation is primarily based on the coupling of a mode guided in the core of the fibre to modes which are propagated in the cladding, have become useful for applications in sensor technology and optical telecommunications, for example for use in band-rejection filters or gain equalizers for optical amplifiers. Unlike short-period gratings, long-period gratings do not backscatter the resonance wavelength.
For making these gratings, a corresponding length of optical fibre is subjected to the action of a radiation source, such as a frequency doubled argon laser, capable of causing a local variation of the refractive index in the core of the fibre. The profile (typically the period) of the variations of the refractive index induced in this way is determined by the characteristics of a writing mask interposed between the radiation source and the fibre. The mask, typically produced by photolithographic or similar methods, is made in such a way as to expose to the radiation, or mask from it, the successive areas of the fibre core in which the refractive index is to be changed or kept unaltered respectively.
In FIG. 1 of the attached drawings, the curve indicated by A represents the propagation characteristics (in terms of attenuationxe2x80x94vertical scale, measured in dB) of a long-period grating made according to the known art. The attenuation is shown here as a function of the wavelength (xcexxe2x80x94horizontal axis, measured in nm) with reference to a non-polarized radiation. The diagram clearly demonstrates the band-rejection filter characteristics shown by the grating in the area lying essentially between 1510 and 1530 nm. The fact that the curve A reproduced here shows an attenuation value of approximately 2 dB even outside the aforesaid area is due solely to the experimental set-up used for plotting the diagram.
On the other hand, the graph of the curve B, relating to the same grating, represents the variation (again measured as a function of the wavelength xcex) of the parameter known as PDL, an abbreviation of xe2x80x9cpolarization dependent lossxe2x80x9d. In gratings made according to the known art, it is usual to find markedly differentiated behaviour in the presence of two optical radiations having polarizations orthogonal to each other. The form of the curve B in FIG. 1 demonstrates, above all, that this difference in behaviour, which is hardly perceptible outside the area in which the grating performs its filtering action, becomes very considerable (showing especially a rather irregular variation as a function of the wavelength xcex) within the area in which the grating carries out its function as a band-rejection filter.
Without wishing to adopt any specific theory in respect of this matter, the applicant has reasons for thinking that this behaviour (which is such that the grating acts to a certain extent as a polarizer) is intrinsically related to the fact that, during the operation of writing the grating, the refractive index of the fibre core loses its characteristic of an essentially scalar value and assumes the characteristics of a tensorial value.
The object of the present invention is to overcome the aforesaid drawback in such a way that it is possible to make long-period in-fibre gratings in which the aforesaid polarization dependence is virtually eliminated.
According to the present invention, this object is achieved by means of a process having the characteristics claimed in a specific way in the following claims. The invention also relates to a device which can be used for the application of this process.